An epidemiological study of records collected by the University of Manchester Children's Tumor Registry in respect of tumors presenting in 1954-73 in children who were under 15 years old and resident in the Manchester Regional Hospital Board area at the time. The study area has a child population of about one million. There is evidence that the cases ascertained (rather more than 2,000) include practically all malignant neoplasms diagnosed in this population. All the clinical and pathological data available for these cases have been reviewed by experts in children's tumors in the interests of diagnostic accuracy. In the present study, these data are being used to explore the frequency and causation of childhood neoplasms with particular reference to: (a) Incidence by histological type, site, age at onset and sex. (b) Distribution in space and time. Possible variations in incidence with personal and family circumstances including incident during the relevant pregnancy. (d) Associated pathology in the children and their relatives. (e) Health of long-term survivors and their progeny.